Separation of pure methionine from methionine nitrile hydrolyzates



Patented Apr. 18, 1950 UNxTED STATES PAiE-NT oFFlcE SEPARATION OF PURE METHIONINE FROM METHIONIANE NITRILE HYDROLYZATES` Robert D. Kral'ovec, 'Wlmingtom` Del., assignor to E. I. du Bont de* Nemours & Company, Wilmingt'on, Del., a corporationrofrDelawarer No Drawing. Application` May-1li, 19.48, Serial No. 27,841

proved synthesis of methionine isdisclose'cl in.

which acrol'ein is reacted with methyl mercaptan in thel presence of charcoal or an amine catalyst and the resulting product is-treated` with HCN to form beta-methylmercaptopropionaldehyde cy.m anhyd'rin. The lattercompound is then subjected to amination by means of' high pressure' ammonia and the resulting alpha-amino-gammamethylmercaptobutyronitrile (hereinafter called methionine nitrile) is hydrolyzed with sulfuric acid to give a reaction product which, upon neutralization with aqueous ammonia or other like alkaline agent, gives a solution containing. methi-. onine. This solution also contains ammonium sulfateV and/or bisulfate (produced byfhydrolysis ofthe nitrile), as well as certain organic by:-

products which vary in composition from .color-x less, volatile, odorous, low-molecular weight compounds to high molecularl weight tar-ry bodies; Some 'of these impur-ities contain. both sulfur andv nitrogen, as does methionine. Some of. the color-1 less, low-molecular weight ley-products give rise to colored higher molecular weight' tars. during the product-Work-up.. These by-products increaser the solubility of methionine in the mixture. Their presence renders theV separation .of pure. methionine in high. yieldextremely ditlicultl and troublesome. These diiculties arev aggravated by the fact that the tarry bodies interfere iiltrations, crvstallizations. andv other similar essential operations in the puri-cation process.

Heretofore. various methods have been employ-ed for separating amino acids from aqueous solutions containing ammonium sulfate or amr. monium bisulfateand. an aminoacid.. Onersuch method, disclosed ire S; PatentfZlZ, isz-to add ammonia (in non-aqueous form ors in the form of an aqueoussolution'), whichcauses pre,-r cipitation of' the ammonium sulfate, thusv producing a solution from` which Ythe amino acid` can bel crystallized'byvaporizingthe solvent. .Attempts have also been madewto'saltl out amine; acidsby .the use of certainsoluble;-inoreanic salts. humm general, these methods have-not been suc-A cessful because soluble vinorganic salts, almost.'

without exception, have beenf found to increaserather thandecrease'solubility of amino acids in Water.r

In the copending application of E. C. Kirk patrick, S. N. 618,781, 4filed September 26,V 1945, now U. S.. Patent 2,443,391. it is'disclosed that pure methoinine can be obtained by heating methionine nitrile with dilute aqueous sulfuric acid, neutralizing the reaction-product to a pH of 5 to 6 by additionof` ammonia, and adding ammonium sulfate' Ge. g. in the formA of recycled mixture containing neutralizing ammonia and ammonium sulfate) directly to the neutralization mixture whereby a. precipitatey containing methionine and coagulated tarry material is formed. The' tarry byproducts in process of application S. N. 618,781, are formed' from substances which, unless removed, interfere with the isolation of pure methionine, and moreover, have the eifect to increasing the solubility of methionine in water. The present invention is concerned with an improved method for removing such impurities from methionine nitrile hydrolysis crudes.

These interfering substances cannot be removed i'rom methionine nitrile hydrolysis products byi extraction with common solvents (petroleum` ether, cyclohexane, esters, ketones, chloroform,Y etc), because 'such extracting media do not possessfhigh solvency for the said Vsubstances.`

An object of this invention is to provide an improved method` for separating pure methionine from the products` of methionine nitrile hydrolysis. Another object is to provide a solvent extraction process. whereby` the separation' of substances which interfere with the separation or pure methionine from` methionine nitrilehydrolysis Vproducts can'be effected.V -A further object is to recover the by-p'roducts of methionine nitrile. hydrolysisy namely ammonium sulfate, ammonia and organic by-products', in a form in which theyv areleconomically handled and' dis'- posed of. Other `objects of the invention appear hereinafter.

Theseandi other objects are accomplished in accordance with the inventionl by extracting anE aqueous solution of` methionine nitrile hydrolysis products preferably with an alcohol solvent: of thel classi' consisting of benzyl alcohol, cyclopentanot, cyclohexanol, methyl cyclohexanol', phenyl.

ethyl alcohol, and n-butanol, and recovering'methionine from the resultant aqueous residue. It'

hasbeen discovered inaccordance with this invernv tionfthatthese specicalcohnls.` are quite unique Closely related straight chain alcohols (n-pentanol, n-hexanol, methoxyethanol, etc.) gave relatively poor results; moreover, cyclohexyl compounds other than cyclohexanol (e. g., cyclohexanone, cyclohexyl esters, etc.) are likewise ineffective. The branched chain higher alcohols, e. g. 2-ethyl hexanol and 3,5,5-trimethyl hexanol, are ineiective, evidently because they do not dissolve the interfering substances. The herein disclosed preferred solvents combine low solvency for methionine with high solvency for the organic impurities, and low solvency with water; these are the characteristics which an extracting solvent must have in the practice of the present invention. In a specific embodiment, the invention is based upon the discovery that variations in pH affect the extractability of these impurities, best results being obtained at a pH of 4 to 7.5.

The invention contemplates the separation of pure methionine from methionine nitrile hydrolysis products by extracting organic substances (other than methionine) from an aqueous solution of methionine nitrile hydrolysis products with the aforesaid preferred alcohols, and thereafter separating methionine from the resultant aqueous residue. This iinal separation of methionine from the residue may be accomplished by any suitable method such as by chilfing the aqueous mixture or by concentrating and cooling the same whereby crystallization of methionine takes place. The optimum temperature for extraction of the impurities from the methionine nitrile hydrolysis products is about 90 to 100 C., although higher or lower temperatures may be employed if desired.

For best results it is generally preferred to control the pl-I at about 4 to 7.5 during the extraction, as stated above. However, the invention may be practiced by extracting the impurities from the acidic hydrolysis mixture without controlling the pH, or without neutralizing any of the acidic catalyst which is present in the mixis cyclohexanol, to assist in removal of the byproducts.

Among the very best solvents for use in practicing the invention are benzyl alcohol, cyclopentanol and cyclohexanol.

The invention utilizes the discovery that by extraction of an acidic or neutralized aqueous solution of methionine nitrile hydrolysis products with the herein disclosed solvents, an aqueous residue is obtained from which methionine, which has relatively little undesirable color or odor, can

easily be obtained. For the preparation of highly purified methionine it is desirable to heat the aqueous residue, after extraction with the solvent, with decolorizing charcoal, whereby a pure, colorless product is obtained.

4When the neutralization step is included in the practice of the invention it is not necessary that the methionine nitrile hydrolysis mixture be bodiments other neutralizing agents such as calcium oxide or calcium hydroxide may be employed. "-When these latter neutralizing agents are used, it is generally desirable to separate '70 neutralized with ammonia, i. e., in particularemf dcalcium sulfate from the resultant mixture by filtration prior to the extraction.

The invention is illustrated further by means of the following examples.

Example 1.-A hydrolysis mixture (17% methionine) obtained by heating crude methionine nitrile with 45% aqueous sulfuric acid at boiling temperature for minutes, was neutralized with i concentrated aqueous ammonia to a pH of 6 and the resulting mixture was diluted with water until the concentration of methionine was 6.7%. The solution thus obtained was heated at a temperature of 90 to 100 C., and was stirred for about 30 minutes with approximately 2 parts by weight of cyclohexanol per part of methionine present. The cyclohexanol layer was then drawn off and the resulting aqueous residue was extracted a second time by the same procedure, using a similar quantity of cyclohexanol. The extracted aqueous solution was concentrated by evaporation until it contained about 10% methionine and 27% (NH4) 2504, and the resulting concentrated mixture was cooled to 10 C. whereby a thick slurry of methionine in ammonium sulfate solution was obtained. This was filtered and the lter cake thus obtained was washed with a quantity of cold water about equal to the weight of methionine present. The resulting moist solid, which contained methionine contaminated with ammonium sulfate, was dissolved in a suiiicient quantity of water to form a 12% solution of methionine. This solution was heated at a temperature of 'to 100 C. with a small quantity of decolorizng charcoal. The resulting mixture was ltered, concentrated to about 20% methionine, cooled to a temperature of about 5 to 10 C. whereby crystallization of the methionine took place. This methionine was removed by ltration in the form of substantially pure white crystalline akes. The ammonium sulfate liquors (rst ltrate and wash) were combined and evaporated to dryness, yielding a brownish white solid which analyzed 92% (NH-02504. The cyclohexanol in the extract was recovered by distillation in a form suitable for reuse in the purication of an additional quantity of crude methionine.

Example 2.-The hydrolysis product (21% methionine) obtained by heating methionine nitrile with45% aqueous sulfuric acid at about boiling temperature for 90v minutes, was neutralized to a pH of about 6 with an ammoniaca] recycle liquor which was produced as described below. The resulting mixture was diluted with the mother liquor obtained in a previous recrystallization of methionine from water, yielding a solution containing about 7.5% methionine, 13% (NH4)2SO4 and 1% by-products. This solution was extracted twice with cyclohexanol at 95 C., the quantity of cyclohexanol employed in each extraction being about 1.5 parts by weight per part of methionine present. The aqueous layer was Withdrawn and concentrated to a methionine content of about 14%. The concentrated solution of methionine was cooled to 0 C. and centrifuged, whereby a moist methionine cake was obtained. This moist cake was pressed to remove most of the ammonium sulfate liquor, and was then redissolved in sufficient hot water to make a 10% solution f After decolorization with charof methionine. coal, this solution was cooled to 0 C. and the resultant precipitate of crystalline methionine 2; In a process for` separating pure methionine.v

from methionine nitrile hydrolysis products, the steps which comprise extracting from an aqueous solutionof methionine nitrile hydrolysis products at a pH of about 4 to '7.5 with cyclohexanol the organic vsubstances other than methionine formed by the said hydrolysis and thereafter separating methioninel from the resultant aqueous residue.

3. In a process for separating substantially pure methionine from methionine nitrile hydrolysis products, the steps which comprise extracting organic by-products of the said hydrolysis from an aqueous solution oi' the said hydrolysis products with an alcohol of the class consisting of benzyl alcohol, cyclopentanol, cyclohexanol, methyl cyclohexanol, phenyl ethyl alcohol, and :n-butanol at a pH of 6 to 7.5 and a temperature 'of about 90 to 100 C., and thereafter separating :methioninefrom the resultant aqueous residue.

4. A process for' separating substantially pure :methionine from methionine nitrile hydrolysis jproducts, which comprises adding ammonia to the said hydrolysis products obtained by heating methionine nitrile with hot dilute aqueous sulfuric acid, the quantity of the said ammonia being sufiicient to raise the pH of the said hydroly- :sis products to from 4 to 7.5, extracting the resulting mixture with cyclohexanol and thereafter separating methionine from the resultant aqueous residue. l

5. A process for separating substantially pure methionine from methionine nitrile hydrolysis products.- which comprises adding ammonia to the said hydrolysis products obtained by heating methionine nitrile with hot dilute aqueous sulfuric acid, the quantity of the said ammonia being suincient to raise the pH of the said hydrolysis products to from 4 to 7.5, extracting the resulting mixture with benzyl alcohol and thereafter separating methionine from the resultant aqueous residue.

I 6. ,A process for separating substantially pure methionine from methionine nitrile hydrolysis products, which comprises adding ammonia to the said hydrolysis products obtained by heating methionine nitrile with hot dilute aqueous sulfuric acid, the quantity of lthe said ammonia being suiicient to raise the pH of the said hydrolysis products to from 4 to 7.5, extracting the re-v sulting mixture with cyclopentanol and thereafter separating methionine from the'resultant aqueous residue.

7.' A process for separating substantially pure methionine from methionine nitrile hydrolysis CII 8 products which comprises neutralizing, ywith, ari aqueous solution containing ammonia and ain-'- monium sulfate, the said hydrolysis mixture obitained by hydrolysis of methionine nitrile with hot aqueous dilute sulfuric acid, the quantity of the ammonia in the said solution of ammonia and ammonium sulfate being Isuicient to raise the' pH of the hydrolysis mixture to from' 6 to 7.5, extracting the resulting mixture with cyclohexa- :no1 at a temperature of about 90 to 100 C., re-

covering methionine from the aqueousl layer re-' sulting from the said extraction, precipitating ammonium sulfate from the mother liquor ob-l tained in the said crystallization of methionine` ,by addition of ammonia, separating the super# natant aqueous solution of ammonia and alm-4 monium sulfate from the said precipitated ammonium sulfate, andfusing the said solution of ammonia and ammonium sulfate for the neutralization of an additional quantity of the said methionine nitrile hydrolysis mixture.

8. A processfcr separating from methionine' impurities obtained by acid hydrolysis of methionine nitrile which comprises extracting the said impurities from the hydrolysis product with an' alcohol of the class consisting of benzyl alcohol, cyclopentanol, cyclohexanol, methyl cyclohexanol,

phenyl ethyl alcohol, and n-butanol at a pH of 2 to 7.5. l

9. In a process for preparing pure methionine, free of the impurities obtained by acid hydrolysis of methionine nitrile, vthe step which comprises' extracting such impurities from the said hydrolysis product with cyclohexanol at a pH of 2 to 7.5.

10. In a process for preparing pure methionine,

free of the impurities obtained by acid hydrolysisy of methionine nitrile, the step which comprises extracting such impurities from the said hydroly-v sis product with cyclopentanol at a pH of 2 to 7.5.

11. In a process for preparing pure methionine, free of the impurities obtained by acid hydrolysis of methionine nitrile, the step which comprises extracting such impurities from the said hydro1y' sis product with benzyl alcohol at a pH of 2 to 7 .5.

ROBERT D. KRALOVEC. REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,071,282 Gluud et a1. Feb. 16, 1937v 2,443,391 Kirkpatrick June 15, 1948 

1. IN A PROCESS FOR SEPARATING PURE METHIONINE FROM METHIONINE NITRILE HYDROLYSIS PRODUCTS, THE STEPS WHICH COMPRISE EXTRACTING AN AQUEOUS SOLUTION OF METHIONINE NITRILE HYDROLYSIS PRODUCTS AT A PH OF 2 TO 7.5 WITH AN ALCOHOL OF THE CLASS CONSISTING OF BENZYL ALCOHOL, CYCLOPENTANOL, CYCLOHEXANOL, METHYL CYCLOHEXANOL, PHENYL ETHYL ALCOHOL, AND N-BUTANOL, AND THEREAFTER SEPARATING METHIONINE FROM THE RESULTANT AQUEOUS RESIDUE. 